Photolithography masks (photomasks) are used in the manufacture of integrated circuits to define circuit features on a wafer. A photomask typically includes a mask substrate having a thin layer of material which defines a pattern of circuit features to be exposed onto the wafer. The pattern layer of circuit features is made of an opaque material which selectively blocks a patterning beam used in photolithography processes. The patterning beam exposes a suitably sensitized film covering the wafer. When the wafer is exposed, the pattern of the photomask is reproduced on the sensitized film. Subsequent processing, such as developing the sensitized film, etching, and the like, in accordance with the particular photolithography technique being used, reproduces the circuit pattern of the photomask on the surface of the wafer to define the desired circuit elements.
When the manufacturing process for the wafer requires multiple masking steps, the subsequent layers to be patterned thereon require the alignment of their photomasks to the patterns on the substrate from the previous masking layers. Therefore, in addition to the circuit pattern on the mask substrate, one or more alignment mark windows are formed in the opaque material of the photomask which have a known position relative to the circuit features in the pattern. The alignment mark windows are used to view features on the wafer called alignment marks. Accordingly, the alignment marks on the wafer are referenced to features on the mask. The alignment mark windows must be large enough to view the alignment marks on the wafer.
Certain photolithography techniques require the alignment marks on the wafer to be directly opposite the alignment mark windows of the mask. Consequently, when the wafer is exposed, the alignment mark window pattern is also transferred to the wafer.
For some processes, transfer of the alignment mark window pattern to the wafer is undesirable. This occurs when the transfer of the alignment mark window pattern destroys the alignment mark on the wafer so that it is no longer useful for subsequent alignment steps.
The transfer of the alignment mark window pattern to the wafer may also be undesirable when the process steps are incompatible with the material on the wafer around the alignment mark. This incompatibility may cause lower yields.
Therefore, a photomask is needed which prevents the transfer of the alignment mark window pattern to the wafer during wafer processing.